The field is two-stroke cycle internal combustion engines. Particularly, the field relates to uniflow-scavenged, opposed-piston engines with air handling systems that provide pressurized charge air for combustion, and that process the products of combustion. In some aspects, such air handling systems recirculate and mix exhaust gas with the pressurized charge air in order to lower combustion temperatures.
A two-stroke cycle engine is an internal combustion engine that completes a power cycle with a single complete rotation of a crankshaft and two strokes of a piston connected to the crankshaft. One example of a two-stroke cycle engine is an opposed-piston engine with one or more cylinders, in which two pistons are disposed in opposition in the bore of each cylinder for reciprocating movement in opposing directions. Each cylinder has longitudinally-spaced inlet and exhaust ports that are located near respective ends of the cylinder. Each of the opposed pistons in the cylinder controls one of the ports, opening the port as it moves to a bottom center (BC) location, and closing the port as it moves from BC toward a top center (TC) location. One of the ports provides passage for the products of combustion out of the bore, the other serves to admit charge air into the bore; these are respectively termed the “exhaust” and “intake” ports. In a uniflow-scavenged opposed-piston engine, charge air enters a cylinder through its intake port and exhaust gas flows out of its exhaust port, thus gas flows through the cylinder in a single direction (“uniflow”)—from intake port to exhaust port. The flow of gas is referred to as the “gas exchange” process. The gas exchange process occurs during that part of the cycle when the intake and exhaust ports are open. For each cylinder of the engine, gas exchange starts at the first port opening of a cycle and stops at the last port closure of the cycle.
In FIG. 1, a uniflow-scavenged, two-stroke cycle internal combustion engine is embodied by an opposed-piston engine 49 having at least one ported cylinder 50. For example, the engine may have one ported cylinder, two ported cylinders, three ported cylinders, or four or more ported cylinders. Each ported cylinder 50 has a bore 52 and longitudinally-spaced exhaust and intake ports 54 and 56 formed or machined in the cylinder wall, near respective ends of the cylinder. Each of the exhaust and intake ports 54 and 56 includes one or more circumferential arrays of openings in which adjacent openings are separated by a solid bridge. In some descriptions, each opening is referred to as a “port”; however, the construction of a circumferential array of such “ports” is no different than the port constructions shown in FIG. 1. In the example shown, the engine 49 further includes two crankshafts 71 and 72. The exhaust and intake pistons 60 and 62 are slidably disposed in the bore 52 with their end surfaces 61 and 63 opposing one another. The exhaust pistons 60 are coupled to the crankshaft 71, and the intake pistons are coupled to the crankshaft 72.
As the pistons 60 and 62 near TC, a combustion chamber is defined in the bore 52 between the end surfaces 61 and 63 of the pistons. Fuel is injected directly into the combustion chamber through at least one fuel injector nozzle 100 positioned in an opening through the sidewall of a cylinder 50. The fuel mixes with charge air admitted into the bore through the intake port 56. As the air-fuel mixture is compressed between the end surfaces it reaches a temperature that causes combustion.
With further reference to FIG. 1, the engine 49 includes an air handling system 51 that manages the transport of charge air provided to, and exhaust gas produced by, the engine 49. A representative air handling system construction includes a charge air subsystem and an exhaust subsystem. In the air handling system 51, the charge air subsystem includes a charge source that receives fresh air and processes it into charge air, a charge air channel coupled to the charge air source through which charge air is transported to the at least one intake port of the engine, and at least one air cooler in the charge air channel that is coupled to receive and cool the charge air (or a mixture of gasses including charge air) before delivery to the intake port or ports of the engine. Such a cooler can comprise an air-to-liquid and/or an air-to-air device, or another cooling device. The exhaust subsystem includes an exhaust channel that transports exhaust products from exhaust ports of the engine for delivery to other exhaust components.
With further reference to FIG. 1, the air handling system 51 includes a turbocharger 120 with a turbine 121 and a compressor 122 that rotate on a common shaft 123. The turbine 121 is coupled to the exhaust subsystem and the compressor 122 is coupled to the charge air subsystem. The turbocharger 120 extracts energy from exhaust gas that exits the exhaust ports 54 and flows into an exhaust channel 124 directly from the exhaust ports 54, or from an exhaust manifold 125 that collects exhaust gasses output through the exhaust ports 54. In this regard, the turbine 121 is rotated by exhaust gas passing through it. This rotates the compressor 122, causing it to generate charge air by compressing fresh air. The charge air subsystem includes a supercharger 110. The charge air output by the compressor 122 flows through a charge air channel 126 to a cooler 127, whence it is pumped by the supercharger 110 to the intake ports. Charge air compressed by the supercharger 110 can be output through a cooler 129 to an intake manifold 130. In this regard, each intake port 56 receives pressurized charge air from the intake manifold 130. Preferably, in multi-cylinder opposed-piston engines, the intake manifold 130 is constituted of an intake plenum that communicates with the intake ports 56 of all cylinders 50.
In some aspects, the air handling system shown in FIG. 1 can be constructed to reduce NOx emissions produced by combustion by recirculating exhaust gas through the ported cylinders of the engine. The recirculated exhaust gas is mixed with charge air to lower peak combustion temperatures, which reduces production of NOx. This process is referred to as exhaust gas recirculation (“EGR”). The EGR construction shown obtains a portion of the exhaust gasses flowing from the port 54 during scavenging and transports them via an EGR loop external to the cylinder into the incoming stream of fresh intake air in the charge air subsystem. Preferably, the EGR loop includes an EGR channel 131. The recirculated exhaust gas flows through the EGR channel 131 under the control of a valve 138 (this valve is also called the “EGR valve”).
In many two-stroke engines, combustion and EGR operation are monitored and optimized based on various measurements related to the amount of charge air delivered to the engine. For example, the ratio of the mass of charge air delivered to a cylinder to the reference mass of charge air required for stoichiometric combustion in the cylinder (“lambda”) is used to control NOX emissions over a range of engine operating conditions. However, in a two-stroke cycle opposed-piston engine with uniflow scavenging, port opening times overlap for a portion of each cycle and some of the charge air delivered to a cylinder through its intake port flows out of the cylinder before the exhaust port is closed. The charge air flowing out of the exhaust port during scavenging is not available for combustion. Thus, a value of lambda based on charge air delivered (“delivered lambda”) to the intake port of a cylinder in an opposed-piston engine with uniflow scavenging overstates the amount of charge air actually available for combustion.
According to priority application Ser. No. 13/926,360, in a two-stroke cycle opposed-piston engine with uniflow scavenging, trapped lambda (λtr) is estimated or calculated based upon the charge air trapped in a cylinder by the last port to close. In this regard, the last port to close can be either the intake port or the exhaust port. Relatedly, the ratio of the mass of charge air trapped in the cylinder by the last port to close (hereinafter, “last port closing”, or “LPC”) to a reference mass of charge air required for stoichiometric combustion in the cylinder is referred to as “trapped lambda”. Since it is the trapped charge air that is available for combustion, a trapped lambda value provides a more accurate representation of the combustion and emission potentials of the engine than a delivered lambda value. A method for determining trapped lambda (λtr) is given in priority application Ser. No. 13/926,360.
Other air handling parameters are used to control various aspects of combustion and EGR operation in two-stroke engines and determinations of their values are based on estimations or calculations that include lambda. For example, burned gas fraction (ratio of burned gas to in-cylinder mass) has a significant impact on the combustion process and thus the emissions of a two-stroke engine. Priority application Ser. No. 13/926,360 discloses a method for determining trapped burned gas fraction (BFtr) using trapped lambda. The trapped burned gas fraction is used to vary the EGR flow rate using an EGR valve to minimize the error between the actual and desired trapped burned gas fraction.
The trapped burned gas fraction provides an important measure of the combustion process and thus of the emissions of an opposed-piston engine. Control of the trapped burned gas fraction can enable an air handling control mechanization to monitor and adjust the combustion process and thereby control emissions as engine operating conditions change. Control of an external burned gas fraction alone does not always provide the degree of precision as may be needed because there can be a significant difference between in-cylinder trapped burned gas fraction and a burned gas fraction based on external EGR. Therefore, in order to control emissions, it is desirable to be able to control the trapped burned gas fraction at all times.
Accordingly, there is a need to improve the accuracy of trapped burned gas fraction control in uniflow-scavenged, opposed-piston engines.